1. Field of the Invention
The present invention relates to an inert-metal lined, steel-body vessels for storage and dispensing of gases; and more specifically, to a removable corrosion free liquid/gas wetted seal for an inert-metal lined, steel-body vessel for the pressurized storage and transfer of ultra-pure gases.
2. Discussion of Related Art
Seamless steel-body tanks are used for the storage and dispensing of pressurized gases for scientific, commercial, and industrial use. These gas storage vessels, which are usually cylindrical in shape and seamless (but can be welded), are of various sizes depending upon the gas to be contained and its use. The most common gas storage vessels are vessels used for storing and dispensing helium or welding gases. The steel body construction of these vessels, allow storage of gases at pressures, which are typically in the range of 1800–5000 psi. The dimension of these cylindrically shaped tanks is roughly a 2″ to 24″ in diameter (OD) and about 2 to 40 feet long. They typically have a flattened bottom end for standing and an internally threaded necked top end through which the gas is dispensed. The internally threaded neck is adapted to receive an externally threaded valve, which can be selectively opened or closed to dispense the contained gas.
For many purposes, the contact of the contained gas with the steel wall of the tank is non-problematic. However, for certain industrial uses, contact of the gas with the steel tank wall is not acceptable. With the advent of more sophisticated processes and products, such as computer chips, where the transistor size is on the order of microns, the specifications on gas purity have become more demanding. Contaminants associated with the interior surface of the steel body of the gas storage vessel such as rust, iron oxide, and dirt compromise gas purity. Further, chemical reactions between the steel body of the gas storage vessel and the stored gas, produce contaminating reaction products, which degrade purity.
To maintain the ultra high purity of the stored gas the interior walls of the steel-body vessel are coated with a relatively inert, corrosion resistant, metal. One such relatively inert, corrosion resistant material is nickel. The preferred method of obtaining this interior nickel coating is electroplating primarily because of economics. Electroplating of the interior tank wall surface is relatively easy, however, plating the internally threaded vessel end closure is problematic. First, extending a uniform coating of nickel over the threaded surface by the electroplating process is difficult. Second, because of strength and coating limitations, the nickel coating should not extend into the threaded region. Failure to extend the coating onto the threads results in exposure of the gas to the steel body surface.
Thus, in order for the nickel to extend into the threaded region, it would be necessary for either the threads to be machined into the nickel coating or for the nickel layer to be deposited over previously machined threads. Machining threads into the nickel layer requires the nickel layer to be very thick, increasing the cost and time of production. There is also a concern about the relatively soft nickel coating having sufficient strength to guarantee that the threads would secure the end closure under service pressures. Furthermore, the bond strength between the nickel layer and the steel body might be insufficient to secure a vessel end closure at service pressures. Alternatively, coating over already machined threads would require the use of a very thin, uniform coating of nickel. It is, however, very difficult to control the thickness of the nickel coating so that it is thick enough to ensure coverage of all the steel but yet not be so thick as to interfere with the engagement of the threads of the end closure.
Problems encountered with screw threads interior the dispensing end of the vessel used to secure a valve is that there is a possibility of particulate contamination from the vessel manufacturing process being caught within the threads. These contaminants could subsequently dislodge and be carried in the gas to the manufacturing process for sensitive scientific or industrial components. Such contamination results in deterioration in component quality and rejection losses.
One attempt to circumvent the above referenced problem is contained in U.S. Pat. No. 6,089,399 issued Jul. 18, 2000. This patent discloses an externally threaded vessel end portion wherein the end portion is sealed by an end closure device having a gas transfer apparatus. An interior threaded securing device threadably engages the threaded exterior surface of the vessel end portion to secure the end closure device. This device however has the inherent drawback of being very expensive to produce and requiring specially machined end securing elements such that standard tankage is not interchangeable. This then requires specialty empty tank exchange arrangements increasing the cost of the gas.
Thus, what is lacking in the art is an end closure device compatible with inert metal lined, ultra-pure gas pressure storage vessels that are internally threaded on the end, such that no steel surface is exposed to the stored gas, is interchangeable as a standard tank, has integrity at service pressures, and is inexpensive to manufacture.